Until Recently, in thermal spraying, it has been the practice to use the highest temperature heat sources to spray metal and refractory powders to form a coating on a workpiece surface. The highest temperature processes currently in use are plasma spray devices, both using an open arc as well as a constricted arc. These extremely high temperature devices operate at 12,000° F. to 16,000° F. to spray materials, which melt at typically under 3,000° F. Overheating is common with adverse alloying and/or excess oxidation occurring. These problems also occur to a lesser or greater degree during the use of the more recently developed HVOF (high velocity oxy-fuel) processes as well as HVAF (high velocity air-fuel) processes. Both of these are combustion type processes utilizing pure oxygen or air containing oxygen as the-oxidizer in the combustion process.
Another prior art method of applying a coating is described in U.S. Pat. No. 5,302,414 Alkhimov et al, issues Apr. 12, 1994, which describes a cold gas-dynamic spraying method for applying a coating of particles to a workpiece surface, the coating being formed of a cohesive layering of particles in solid state on the surface of the workpiece. This is accomplished by mixing powder particles having a defined size of from 1 to 50. microns entrained in a cold high pressure carrier gas into a preheated high pressure gas flow, followed by accelerating the gas and particles into a supersonic jet to velocities of 300 to 1000 meters per second, while maintaining the gas temperature sufficiently below the melt temperature so as to prevent the melting of the particles. In the operation of this cold gas-dynamic spraying method there are a set of critically defined parameters of operation (particle size and particle velocity for any given material) which makes the process very sensitive to control while maintaining consistent coating quality as well as maintaining useful deposit efficiencies. In addition, the cold gas dynamic spray method as described by Alkhimov et al, is limited to the use of 1-50 micron size powder particles.
Another prior art method of coating is described in U.S. Pat. No. 6,139,913, Van Steenkiste et al, which describes a kinetic spray coating method and apparatus to coat a surface by impingement of air or gas with entrained powder particle in a range of up to at least 106 microns and accelerated to supersonic velocity in a spray nozzle and preferably utilizing particles exceeding 50 microns. The use of powder particles greater than 50 microns overcomes the limitation disclosed by Alkhimov et al. Van Steenkiste et al, while utilizing the same general configuration of the prior art in which the cold high pressure carrier gas with entrained powder material is injected downstream of the heating source of the main high pressure gas into the heated main high pressure gas overcomes the limitations of Alkhimov et al by controlling the ratio of the area of the powder injection tube to 1/80 relative to the area of the main gas passage. By controlling this ratio, it limits the relative volume of cold carrier gas flowing into the heated main gas flow, thereby causing a reduced degree of temperature reduction of the heated main high pressure gas. The net temperature of the main high pressure gas when mixed with the carrier/powder gas flow is critical to determining the velocity of the gas exiting the supersonic nozzle and thereby to the acceleration of the powder particles. As indicated by Alkhimov et al, a critical range of particle velocity is required in order that a cohesive coating is formed. The particle size, the net temperature of the gas and the volume of the gas determine the gas dynamics required to produce a particle velocity falling into the critical particle velocity range.
The cold gas dynamic spray method of Alkhimov et al is limited to the use of a particle size range of 1-50 micron. This limitation has been found by Van Steenkiste et al to be due to the heated main high pressure-gas being cooled by injecting into it the cold high pressure carrier gas/powder. Because of the reduction in gas temperature, the maximum gas velocity that can be achieved is too low to accelerate powder particles larger than 50 microns to the critical velocity required to achieve the formation of a cohesive coating buildup. Van Steenkiste el al improves on this by limiting the amount of cold high pressure carrier gas being injected into the heated high pressure main gas by defining the ratio of the cross sectional area of the bore of the powder injection tube to the area of mixing chamber. This limited the proportion of cold carrier gas mixed into the heated main gas thereby reducing the degree of temperature reduction of the heated high pressure main gas, which then allows for higher gas velocities to be achieved. This provides the ability to accelerate larger particles of a size range greater than 50 microns to a velocity above the critical velocity required to form a cohesively bonded coating buildup. However, the kinetic spray coating method and apparatus of Van Steenkiste et al state an upper limit of the particle size range 106 microns, based on experimental results.
In addition in Alkimov et. al. the main gas is heated upstream of the nozzle, then just upstream of the throat of the nozzle, they introduce the particles and cold carrier gas which lowers the final temperature of the combined main gas/carrier gas/particles. This causes the velocity of the particles to be slower than if the temperature of the main gas was not reduced. Accordingly, in Alkimov a much higher main gas temperature must be used to accommodate the cooling effect of the introduction of the cold carrier gas and particles. With standard electric heaters, the main gas temperature can only be increased to 1300 to 1400 degrees Fahrenheit. This limits the velocity of the particles and hence the size of the particles that produce cohesively formed coatings. Although the pressures of the gases can be increased to increase the velocity of the particles this also increases the complexity and the expense of the system. Accordingly Alkimov is limited to particle sizes of 1 to 50 microns.